Gas exchange and ¹³C-based estimates of intrinsic water-use efficiency show different responses to CO₂ enrichment: a global meta-analysis of experimental studies

Tree intrinsic water-use efficiency (iWUE) is a key characteristic of ecosystem functions central to the global cycles of carbon, water, and energy. Gas exchange and stable carbon isotope compositions have been widely used to assess iWUE. However, it remains unclear to what extent the iWUE estimates obtained using different approaches are comparable, particularly in combination with changes in temperature, precipitation, and experimental duration. Herein, the responses of different estimators of iWUE (as A/g(s) or through isotopic composition of leaves or tree rings) to elevated CO2 concentrations (eCO(2)) were assessed through a meta-analysis of 361 observations from 98 independent experiments. Our results show that (1) different proxies for iWUE provide inconsistent estimates of the response of iWUE to eCO(2). (2) Throughout the growing season, the average response of g(s) to eCO(2) was smaller than the single-point values estimated by gas exchange and the eCO(2)-induced reduction in g(s) was stronger for evergreen species than for deciduous species. (3) Precipitation, rather than temperature, significantly affected the responses of C-13-based estimates of iWUE to eCO(2). (4) There was no significant response of leaf C-13-based iWUE estimates to eCO(2). (5) The importance of photorespiration in elevated CO2 enhancement responses may be overestimated in evergreen species.